If you want to use this to set the height of a FontString object after setting its text, you will need to make sure the initial size of the FontString object is big enough to allow the text. EXAMPLE: If the object will fit 2 lines, and you set the text to a 4 line string, the string will be truncated with "..." when it is set by SetText(), and this function will return the height of 2 lines of text instead of 4.

MyFrameText:SetHeight(1000); -- Arbitrary number, this is about 83 lines of text, assuming ~12 pixels each
MyFrameText:SetText("This\nIs\Four\nLines");
MyFrameText:SetHeight(MyFrameText:GetStringHeight());
print(MyFrameText:GetHeight(); -- We will see that the new frame height is about 48.